The present invention relates to hot melt adhesives, and more specifically to hot melt adhesives having improved hydrophilic properties and which find usefulness in the manufacture of disposable nonwoven articles.
Nonwoven fabric is comprised of an interlocking fiber network, and is employed in the construction of disposable goods. Specific applications of nonwovens have included disposable diapers, sanitary napkins, surgical drapes, hospital pads and adult incontinence products.
In such applications it is generally necessary to adhere nonwoven, tissue, absorbent fluff or the like to another substrate. This second substrate may be another nonwoven fabric, tissue, or a material such as a polyolefin e.g. a polyethylene or polypropylene layer. Typically, a hot melt adhesive has been used to bond such materials together since there is no evaporation step necessary during manufacture, as would be the case for water-based or solvent-based adhesives. Suitable hot melt adhesives must possess the appropriate bond strength to adhere the substrates involved, and must also possess good flexibility, no staining or bleed through, suitable viscosity and open time to function on commercial equipment, acceptable stability under storage conditions, and acceptable thermal stability under normal application conditions.
Many different polymers have been used in hot melt adhesives employed in the construction of disposable nonwoven goods. In this regard typical hot melt adhesives have employed polymers which have included S-I-S (styrene-isoprene-styrene); SBS (styrene-butadiene-styrene); SEBS (styrene-ethylene-butylene-styrene); EVA (ethylene vinyl acetate); and APAO (amorphous poly alpha olefin). While these polymers, when properly blended, provide acceptable adhesion between most substrates employed in typical nonwoven construction such as diapers, and further provide acceptable adhesion under dry conditions, they have had several shortcomings which have detracted from their usefulness.
One of the most noteworthy shortcomings of prior hot melt adhesives concerns the manner in which the adhesive, which is typically very hydrophobic, reacts when exposed to liquids, such as water, urine, or the like. Normally, one would expect the hydrophobic character of hot melt adhesives to be an advantage since such adhesives will provide good dry bonds and will normally maintain an acceptable bond strength when wet. However, manufacturers of disposable nonwoven articles such as diapers have endeavored to produce products which are much thinner in their overall thickness and profile and which incorporate super absorbent materials in place of fluff, which is normally in the core. Thus, it is extremely important in such nonwoven constructions to insure that water, urine or other water-based discharges or solutions are directed toward the absorbent core as quickly as possible, and that any material that might hinder such action be eliminated or at least minimized. As a result, one can now readily understand why the hydrophobicity of typical hot melt adhesives is undesirable since it is a characteristic which inherently hinders fluid transfer into the core of such articles.
Therefore, it has long been known that it would be desirable to have a hot melt adhesive which is useful for bonding to substrates which are typically employed in the construction of nonwoven articles, such as polyethylene, polypropylene, nonwoven, tissue, or fluff, and which further maintains acceptable wet bond strength following exposure for prolonged periods of time to water, urine or similar materials. At the same time, such adhesives should be more hydrophilic to not hinder fluid transfer into the absorbent core of such articles.